The brief
The Golf's ABS warning light had come on and stayed on, and the traction control had disabled itself in the process. The owner could feel the difference under harder braking, no electronic intervention. He brought it in.
The ABS reads how fast each wheel is turning from a small sensor at the hub. When one of those signals fails, the ABS can't trust its picture of the road, so it puts up the warning and steps back, and the traction control, which uses the same readings, drops out with it. A warning that comes on and stays on, rather than flickering, usually means the sensor has failed outright.
The diagnosis
On the scanner the codes pointed at one sensor, the left front wheel speed sensor, with a string of faults logged against it, from open circuit to mechanical malfunction. A meter check at the connector confirmed the sensor itself was the failed component, with the harness and the tone ring it reads off both fine.
That makes it a sensor replacement, and since the right front sensor is the same age and the same wear item, the call was to do the front pair rather than be back for the second one soon.
The work
The car went up, the front wheel speed sensors were unbolted from the hubs and new VAG-spec sensors fitted with fresh O-rings, the harness clipped back into its proper run so nothing rubs. Then the stored faults were cleared from the brake module and the system checked over.
A road test with a series of braking events confirmed nothing came back.
The outcome
ABS warning off, traction control back online, and no codes after a full drive cycle.
The Golf went home with the braking electronics working properly again. The ABS and traction control are there for the moment you need them, so getting the failed sensor changed, the pair refreshed and the system back online put the safety net back where it should be.